The present disclosure is directed to electrodes for galvanic electrochemical cells (cells that produce direct current, D.C. electricity from stored chemical energy) for secondary (rechargeable) batteries that store redox chemical energy by cycling molecular valence contained within faradaic material coatings of graphene structures (e.g. carbon nanotubes, CNT) or within faradaic particles entrained within such structures particles to produce or accept D.C. electricity, fuel cells that catalytically convert hydrogen or hydrocarbon fuels and oxygen to D.C. electricity, oxygen breathing secondary batteries and capacitors that store D.C. electrical energy as charges on and/or in surfaces.
Batteries have for many years continuing to the present day used pastes comprising active faradaic particles, carbon powder additives for charge conduction and exchange with metal and binders, usually PTFE suspension, to durably coat the material onto separators that keep polar electrodes from mutual contact or onto metal surfaces otherwise separated in a unit cell, defined as one pair of separated positive and negative electrodes. There have been may published advances in galvanic properties of particles used in rechargeable batteries but not in the architecture just described that uses them in pastes. Goodenough The Li-Ion Rechargeable Battery: A Perspective, J. Am. Chem. Soc., 2013, 135 (4), pp 1167-1176, “Chemists are contributing to incremental improvements of the conventional strategy.[redacted] . . . while retaining a Li+ conductivity σLi>10−3 S cm−1 . . . ”, i.e. Siemens/cm or 103 Ω-cm. ‘Conventional’ strategies use pastes having thickness of about 100 μm so electrode resistance is ≈10 Ω-cm2 which means 10 ohms resisting the flow of electricity through 1 cm2 of electrode area. It is understood that there are two main sources of this resistance. One is ohmic resistance to electron flow between surfaces where charge transfer is created and terminals required of any electrochemical cell. The second, in series with the first, relates to ion mobility and the kinetics of their obligatory ‘flow’ between electrodes in batteries and fuel cells. This also appears as resistance in impedance measurements.
Gaberscek, The Importance of Interphase Contacts in Li Ion Electrodes: Electrochem. Solid-State Lett., 11, A170 (2008), published a detailed study of resistance in the Li-ion battery confirming 10 ohm-cm2 and attributing most of that to electric charge transfer to metal current collectors at low discharge or charge current where ion kinetics has less influence on measurable resistance. He disproves the commonly assumed strong influence of an SEI layer but shows increasing resistance with current density due to retarded ion mobility. Electrolyte contact with active faradaic sites accounts for much of ion barrier resistance. Membrane ion permeability and diffusion through electrolyte account for the remainder of ion mass transport impedance. These important details will be appreciated in further disclosure of the instant invention.
Li-ion cells show consistent real axis resistance of 50 mΩ (milliohm) for cells having ≈200 cm2 area electrodes. Again, that is 10 Ω-cm2 for the entire collection. i2R heat can be related to the difference in charging and discharge energy to estimate R as Ω-cm2:R=(Vc−Vd)*(Ah/g)/(id)2(g/cm2)Where (Vc−Vd)* is average charge to discharge voltage differential and id is the discharge current/gram of the particle. (Ah/g) is the measured capacity over the correlated voltage range. Vc/Vr for very low charging current, where Vr is battery rated open circuit voltage and influence of internal impedance is negligible, is an irreducible efficiency for most faradaic electrode couples. It can be as high as 97% to 98%. The (Vc−Vd) decrement for useful discharge current produces a loss of efficiency due to resistance within the battery.
Published charge/discharge voltage profiles as a function of ampere-hour capacity can be used to compute resistance values associated with specific examples. Ma, High Rate Micrometer Ordered LiNi0.5Mn1.5O4, Journal of The Electrochemical Society, 157 (8) A925-A931 (2010), tested High Rate Micrometer Ordered LiNi0.5Mn1.5O4 spinel particles as advanced Li-ion cathode material in conventional paste form using unusually high concentration of carbon black and PTFE binder to limit electrode resistance for the purpose of extracting mass based energy density of the spinel; It was realized by using 15 wt.-% to as much as 65 wt.-% carbon black at the expense of volume energy storage density to remove, as much as possible, “rate limitations”. Even so, resistance is still in the range of ½ to 1 ohm-cm2. Commercial batteries cannot afford to focus on such a limited metric since value is more a matter of volumetric energy storage density. On that basis there is more emphasis on active material weight/volume at the expense of conductive additives to the paste. Their published charge/discharge profiles compute to 10 ohm-cm2 as measured directly by Muenzel, Valentin et al., A Comparative Testing Study of Commercial 18650—Format Lithium-Ion Battery Cells, Journal. ECS, 162 (8) A1592-A1600 (2015), for additional confirmation. Consequently, actual commercially available batteries must use high electrode area, limited area current density (10 to 30 mA/cm2), and limited discharge capacity, ≈2 C and long recharge time due to low current density.
Capacitors that depend upon a surface ionic double layer (supercapacitors) involve ion migration within electrolyte but do not require ion exchange between electrode materials. Their chief source of resistance is electrical contact with a metal current collector or cell terminal which has a profound influence on power density and a more subtle influence on energy density. Paste coatings and more advanced nanostructured carbon layers on metal current collectors continue to suffer from contact resistance of about ½ ohm-cm2 even as thin layers. Higher capacity is spread over large area similar to battery architecture. A method of attaching nanocarbon structures to metal surfaces of the instant invention provides uniquely high capacitance almost independent of layer thickness.
Prior art fuel cell electrodes continue to use membrane electrode assemblies (MEA) that depend upon solid state electrolyte ionomers, carbon and noble metal catalyst paste mixture coatings having both high ohmic and ion mass transport kinetic resistance. Separating membranes commonly used in prior art fuel cells have 10 times the ion flux permeability in the plane of the membrane than through it where it is needed. Very high resistance can be accorded to that fact alone. When added to ohmic resistance ordinary fuel cell current density is never much greater than ½ A/cm2.
Most advanced materials combine carbon nanotubes, CNT or related graphene structures with nanoscale faradaic materials in batteries and pseudo-supercapacitors but a problem of effectively connecting these electrically with low ohmic resistance to current collectors or any metal surface persists in the prior art which shows no similar examples for fuel cells. Structures that compress pastes to secure them as bound to respective sides of separating membranes usually require the membrane to be selectively permeable to at least one of the ions that must be exchanged between electrodes for electrical current to flow. Such membranes tend to retard ion kinetics and are not completely reliable for use in repeated charge cycling. Electrode current density is further limited by diffusion characterized by time related parameters that vary inversely as the square of the diffusion path length and is the reason for typically thin electrodes and large electrode area. These compounding limitations become balanced for fuel cells at about ½ Amp/cm2 or, in prior art batteries and supercapacitors, much less.
It must be noted that the current density limitation above described is not so much controlled, especially in fuel cells, by a Tafel slope and overvoltage characteristic of the Butler-Volmer equation but by the value of electrode exchange current density io, i.e. coefficient in that equation. Absent any other limitation, io is a function of galvanic particle mass concentration more than chemical process limitations. Prior art mass concentration of active faradaic material per unit projected electrode area is related to metal coating thickness or faradaic material volume-%. It also depends upon the ratio of active material area exposed to electrolyte per unit projected electrode area. Some advanced and emerging galvanic particles, even in the size range of 0.1 to 10 μm have exposed area ratios comparable to nanoscale particles of similar net weight so their actual size is no longer much of a factor. In prior art, when these particles are applied to metal current collectors as paste coatings containing 5 to 55% of active material volume, the rest comprising carbon and binder, electrical resistance in the path between particle and metal increases with active particle volume and weight ratio. The relationship forces a limiting tradeoff between energy storage capacity and power density. It is resistance that limits area specific current density and useful io. One can design io to reach 10 Amps/cm2 but resistance of ½ ohm-cm2 is what makes that impossible because the iR voltage loss exceeds the voltage produced.
High area concentration of SWCNT or MWCNT (single and multi-wall carbon nanotube) here designated CNT and other graphene structures can be grown or deposited on metal surfaces and coatable in some cases with faradaics in the most advanced materials. Discharge would be virtually resistance free but for the same problem that continues to plague batteries, namely, electrical resistance in a connection of CNT fiber ends to metal where they are ostensibly attached. Carbon structures attached as formed on metal tend to peel off due to volume change in the material when exposed to charge/discharge cycles in fixed electrolyte; which testifies to their feeble electrical connection when ‘attached’; Asari, USPTO Pub. No.: US 2010/0086837 A1, Apr. 8, 2010. Even if attachment issues are addressed by scoring and compressing the CNT layer, electrical resistance remains.
An especially desirable material and one widely publicized as breakthrough technology for batteries and supercapacitors is single or multilayer graphene sheet, CNT precipitated as woven or nonwoven cohesive mats, i.e. pellicles on metal current collectors. These have been coated with both cathodic and anodic nanoscale faradaics on their graphene surfaces; potentially a very effective formulation for batteries if they could be attached to metal current collectors with less resistance. Gold coatings were used by Nano-Lab, www.nano-lab.com/buckypaper to improve electrical contact of cohesive non-woven CNT with metal but reported 0.1 Ω-cm2.
There have not been reliable examples of high electrical current density or voltage in batteries, capacitors or most galvanic electrochemical cells in actual use. Heretofore, electrical current density (amp/cm2) in electrochemical cells that produce direct current electricity from stored chemical energy (batteries) or stored charge (capacitor) has been limited to substantially less than 0.1 amp/cm2 referred to projected electrode area. The inventor has measured the resistance of many samples of these materials and finds it consistent with literature that deals with the subject to be about ¼ to ½ ohm-cm2 also referred to the projected electrode net surface area. That means 1 amp/cm2 will produce a loss of ¼ to ½ volt in cells that barely generate 1 volt. The relationship accounts for low current density being accepted as a universal barrier.
Asari US 2010/0086837 describes a method for scoring a surface layer of CNT's attached at one end to a metal surface for the purpose of preventing the layer from becoming detached from the metal because of charge/discharge cycling. CNT is grown (by a CVD process) attached at one end to the metal and used in this form as capacitor galvanic material. Both physical attachment and electrical connection remain problematical and this form is not used as a galvanic pellicle in this invention. Sassin, M. B. et al, Redox Deposition of Nanoscale Metal Oxides on Carbon for Next-Generation Electrochemical Capacitors, Accounts of Chemical Research, Oct. 26, 2011, uses it to show how strongly coupled asymmetric coatings of MnO2 and Fe on the surfaces of the CNT can greatly increase pseudo-supercapacitance and charge voltage but CNT attachment to metal remains too unstable for acceptance as electrodes by industry.